Stainless steel and method



May 2, 1950 G. N. GOLLER STAINLESS STEEL AND'METHOD Filed Sept. 6, 1946GEORGE N. GOLLER www Patented May 2, 1950 UNITED STATES PATENT FFICEvSTAINLESS STEEL AND METHOD George N. Goller, Baltimore, Md., assignor toArmco Steel Corporation, a corporation of Ohio Application September 6,1946, Serial No. 695,216

(Cl. 'i5-124) 11 Claims.

This invention relates to chromium-nickel stainless steels, moreespecially to a method for providing the steels in the hardenable andhardened conditions, and to the resulting products and manufactures.

An object of my invention is the provision of chromium-nickel stainlesssteels which are amenable to hardening by heat treatment from a soft,formable and maehnable condition.

Another object is the provision of a reliable and commercially practicalmethod for hardening stainless steels of the character noted.

A further object of my invention is the provision of a direct and highlyeffective method for producing chromium-nickel stainless steels to acondition suited for fabricating operations such as those including,cold-rolling, drawing, stamping, punching, upsetting or machining, andto a strong hardened condition by subsequent heat treatment.

A still further object is the provision of hardened, strong andwear-resistant `chromium-nickel stainless steels and articles thereof.

Other objects of my invention will be obvious and in part pointedouthereinafter.

The invention accordingly consists in the combination of elements,composition of materials, and features of production and in the variousoperational steps and the relation of each of the same to one or more ofthe others as described herein, the scope of the application of which isindicated in the following claims.

The single ligure of theaccompanying drawing graphically representsproportions of chromium and nickel which are at times employed in thecomposition of my stainless alloy steel, as will be pointed out morefully hereinafter.

As conducive to a clearer understanding of certain features of myinvention, it may be noted at this point that stainless steels bydefinition are carbon steels which include about to 35% chromium, withor without nickel. and with or without supplemental additions of copper,manganese, silicon, cobalt, molybdenum. aluminum, tungsten, vanadium,titanium. columbium, sulphur, and the like, for special purposes, and aremainder which is substantially all iron. The carbon content usually islow, this being on the order of 0.02% to 0.20%, although it may behigher for particular needs.

In the above group of steels, there are certain chromium-nickelvarieties, especially those which are classiable with the more commonlyknown 18% chromium-8% nickel grades, which remain stably austenitic atroom temperatures after quenching from annealing temperature, and in nomanner are hardenable by heat treatment. These steels arework-hardenable, .but of course the property of work-hardenabilitycannot always be relied upon for the satisfactory provision of hardenedproducts. In view of other properties, the steels are in widespreaddemand for fabrication by such methods as hot-working, cold-forming,machining, punching, drilling, drawing or spinning. After fabrication,the steels are put into use either in soft annealed condition, or inworkhardened condition where feasible.

Under certain circumstances, some few of the chromium-nickel stainlesssteels have been known to respond to hardening heat treatment, this byvirtue of the addition of one or more precipitation-hardening agentssuch as titanium, columbium and copper. 'The use of those hardeningmaterials has depended upon a well studied proportionment of alloyingredients in the steel, followed by a critical form of heat treatmentto eect the hardening. Columbium and titanium, however, are relativelyexpensive materials. The chromium-nickel-titanium or columbium stainlesssteels, moreover usually contain stress-laden ferrite as an essential tohardening from the annealed condition. When the steels are produced tosuch composition as to present a comparatively soft, substantially fullyaustenitic structure at about room temperature after annealing, they aremore amenable to cold-working and forming operations, but the extent ofthe hardening treatment usually is sacriced.

In the production and treatment of certain stainless steels,.the elementaluminum heretofore has been recognized as a deoxidizing material andalso one which frequently serves for developing an aluminum-containingoxide film on the steel surface which is resistant to heat andcorrosion. The alloying of aluminum with suitable quantities of otherincluded elements of the steels is known to impart high'temperatureproperties, this at times being sufficient to justify the productionfrom the metal of such articles as heat engine valves, turbine blades,and the like. In many of the stainless steels, aluminum is said byprevious investigators to exert an impairing eiect upon the ability toharden. I iind that this by far is not true of all stainless steels, forunder certain circumstances the aluminum, which is a relatively cheapmaterial, serves in a highly benecial manner as a precipitationhardening agent.

An outstanding object of my invention accordingly is the provision ofchromium-nickel-aluminum stainless steels which are readily fabricatedinto a host of different products having hardening characteristics attemperatures sufficiently low to avoid substantial scaling anddistortion due to heat, and which steels and products fashioned thereofare characterized by other desirable properties in both the prehardenedand hardened conditions.

Referring now more particularly to the practice of my invention, Iprovide an especial quality atoaves of chromium-nickel stainless steelwhich, through the close correlation of amounts of chromium and nickel,with critical amounts of the ingredients aluminum and carbon, in thecomposition thereof, is highly suitable for fabrication in therelatively soft condition and for precipitation hardening by heattreatment to a corresponding condition of increased tensile strength andyield strength. In the practice of my invention, I provide stainlesssteel in which the chromium and nickel contents are in substantialaccord with the abscissa and ordinate of any given point of area ABCD inthe accompanying diagram, and which further contains anywhere from about0.02%,to 0.12% carbon, aluminum from about 0.50% to 2.50%, fromincidental amounts up to about 8.0% manganese, from incidental amountsup to approximately 2.0% silicon, with or without molybdenum ranging upto about 3.0% illustratively 'to enhance corrosion resistance of thesteel, and the remainder substantially all iron. In this, however,should the carbon, aluminum, silicon, or manganese content be somewhatdifferent from the amount upon which the accompanying diagram is based(the basis of the diagram being amounts of actual chromium and nickel ascalled for, about 0.06% to 0.08% carbon, 0.60% to 1.0% aluminum,incidental amounts of manganese and silicon up to 1.0% each, sulphur andphosphorus up to 0.040% each, and the remainder substantially all iron)or should molybdenum be used, I find it preferable to modify thechromium or nickel content of the steel, or both of the contents as thecase may be, so as to achieve chromium-like and nickel-like componentsin the steel which are substantially equivalent respectively in ferriteand austenite-forming relation to those amounts of chromium and nickeland of the other elements represented in'the diagram. For example, Ioften replace a part of the chromium called for in the diagram with aquantity of aluminum, silicon or molybdenum, the replacement beingapproximately on a 1 to l ratio with respect to chromium, and for suchpurpose as maintaining a desired relation between the austenite andferrite forming components of the steel substantially as would beachieved by rigid adherence to the diagram. Similarly, I occasionallyadd several or all of the replacement elements in partial substitutionfor the chromium. The practice of substitution I find is particularlyadvantageous where either the aluminum or silicon is to exceed about1.0%, or where molybdenum is a constituent of the steel. Followingsubstitution, the actual chromium content of the steel may at times besomewhat below those amounts prescribed by area ABCD in the diagram andstill, in view of the eiIect of the substituted element or elements, bein substantial accord with the area.

Where the permissible quantity of carbon exceeds about 0.08%, or wheremore than small amounts of manganese are present (say for exampleamounts in excess of about 1.0%) I usually employ a proportionallydecreased quantity of nickel in the steel as compared with theaccompanying diagram. The actual nickel content of the steel, however,importantly is not less than about 3.5% after substitution. Should thepermissible quantity of carbon be on the low side, i. e. below about0.06%, I usually increase the nickel content as compared with thediagram, or evenat times instead increase the manganese content. Foreach part of nickel I add'about 2 parts manganese or on the order ofabout 1/zo to 1/am part carbon as the substantial equivalent.

The actual nickel content of the steel, after partial substitution,consequently may on occasions fall even considerably outside thoseamounts prescribed by area ABCD in the diagram or remain inside andstill, in View of the substituted element or elements and thecontributed effect thereof, be in substantial accord with the area.

Where desired the steel contains such addition elements as sulphur and/or selenium in amounts sufficient to enhance free-machining properties.The value of such elements is realized especially in machining the steelin the annealed state.

In accordance with the preferred practice of my invention, I produce astainless steel containing chromium and nickel in amount substantiallyin accord with the abscissa and ordinate respectively ol any given pointsubstantially falling within area abcd in the accompanying diagram, theamounts advantageously being about 16.65% to 17.5% actual chromium andapproximately 6.75% to 7.5% actual nickel as represented by the areaabcd. The steel, also preferably contains, on the basis of the diagram,aluminum in amounts between about 0.60% and 1,0%, carbon within theapproximate range of 0.06% to 0.08%, and the remainder substantially alliron. There are preferably small amounts of such elements as manganese,silicon, sulphur and phosphorus in the steel, as for example manganeseand silicon each not exceeding about 1.0% and sulphur and phosphoruseach up to about 0.040%.

I condition the steel, either as wrought or cast, for subsequent formingand fabrication and for hardening of the formed and fabricated products,by heating the same in a temperature range preferably not lower thanabout 1800 F. and extending up to about 2000 F. for such period of timeas to place the metal in an austenitic aluminumsoluble condition whichis retainable down to at least about room temperature. This heating isin the nature of an annealing treatment. To achieve the annealing, Iusually adjust the met-al to temperature and maintain or hold the sameat temperature as in a suitable heat treating furnace. The holding timeas applied to the steel is not too critical. About one-half hour isquite satisfactory from the standpoint of economy and of ensuringsolubility of the aluminum.

With the steel in aluminum-soluble austenitic condition, I discontinuethe annealing operation and quench the heated metal as in air, oil orwater, conveniently to around room temperature. After the quenching, thealuminum constituent remains in solution. The metal is of soft,-substantially full," austenitic quality characterized by ductility,hardnesses usually below about Rockwell B-92, is formable, andmachinable, one or more of such properties make it possible at thispoint to effect fabrication into any of a host of prehardenedchromium-nickel stainless steel products.

From my annealed and quenched steel, which then is in the prehardenedcondition and capable of being hardened by subsequent treatmenthereinafter described, I provide products in shapes which are less basicor more intricate, illustratively structural members for aeroplanes, asthose parts requiring great strength from the standpoints of yield intension and compression coupled with corrosion resistance and toughness:coldheaded bolts and screws, as for example those eventually to includehardened Shanks; surgical instruments including those in the eld ofdentistry; valves and valve seats; die blocks; ilxtures and trim. Inthese, I take advantage of the exnickel-aluminum stainless steel to apreliminary hardening heat, this being a, reheat considered in the lightof the already completed annealing treatment. Therefore, topreliminarily harden the steel, I reheat it, illustratively in the sameheat treating furnace employed for the annealing, this time up to withinthe approximate temperature range of 1200" F. to 1600 F., preferably upto about 1400" F., and there hold the metal to desired temperature ortemperatures for ap-` proximately A hour or more. During the holdingoperation, as for example with the steel at around 1400 F., thereresults a precipitation of carbides, possibly some precipitation ofaluminum compounds, and a raising of the transformation point of theaustenitic matrix. The steel, upon cooling, consequently transforms toinclude a "soft martensite-like constituent. The transformation occursat quite low temperature such as between about 175 F. and 225 F. Thus,at the completion of the reheating stay, I quench the steel as in air,oil or water with the result that transformation does occur above roomtemperature to achieve a` partial hardening of the metal, or of thearticles made thereof as described hereinbefore. Hardnesses after thistransformation vary in or near the range of Rockwell C-24 to C-28depending upon the particular composition and the conditions oftreatment within the limits noted.

My chromium-nickel-aluminum stainless steels retain sufficient softnessand ductility in the preliminarily hardened, transformed condition topermit working illustratively by cold rolling, or drawing and still aremachinable and capable of being fabricated through cutting, punching,drilling and the like. For this reason. and because the metal can befurther hardened without phase transformation and changes in dimensionsusually accompanying transformation, I frequently put off all or part ofcertain forming and fabricating operations to be accomplished on thesame until after the preliminary hardening treatment, and then produceany of a wide variety of chromium-nickel-aluminum stainless steelarticles, as to achieve substantially finished dimensions.

When transformation and preliminary hardness have been achieved throughreheating and quenching, as hitherto described, and following theperformance of forming and fabricating operations before or after thepreliminary hardening treatment, should the operations be desired, thesteel then is ready for additional hardening,

sumciently low to avoid scaling and warping of the metal due to heat andconstitutes a further part of my entire hardening treatment. Ineffecting the additional hardening, I heat the alloy metal, as in theform of fabricated products, within a low temperature range of about 750F. to approximately 1000 F., preferably at about 900 F., and hold thesame at temperature for about one hour. The time of holding undertreatment, howevenmay vary from approximately one-half hour to some twohours or more with satisfactory results. About one hour is preferred.The treatment serves to precipitate aluminum as a compound and imparts amaterial gain in hardness to the alloy metal. This hardness, I find,upon quenching the steel as in air, oil or water from the hardeningtemperature, comes within the approximate range of C-39 to C-46Rockwell. The exact phenomena which takes place during thisprecipitation hardening treatment are not so clear although aluminum insome form is precipitated. From indications of dilatometer tests nophase transformation or' substantial changes in volume of the metal areto be noted during the hardening reaction. It is believed that thereaction may involve some rearrangement or ordering of the latticestructure of a precipitated nickel-aluminum compound within the lattice`structure of the matrix which thereafter exerts an interferencehardening effect. This belief, however, is not fully conrmed and,therefore, I do not wish to be bound by the same.

In the hardened condition achieved through quenching from thehereinbefore mentioned reheat at 750 F. to 1000 F., the so-conditionedsteel or steel products are very strong having a high yield strength intension and compression, are substantially free from directionality, andare quite resistant to corrosion. By virtue of the low temperatures atwhich the preliminary hardening treatment and especially the additionalhardening treatment are conducted, the steel emerges substantiallywithout heat scale and unwarped by heat. The hardened steel usually hasa straw surface color.

In way of illustration, a chromium-nickelaluminum stainless steelprovided having a composition falling within the `heretofore notedgeneral range, and more specifically containing approximately 0.07%carbon, 16.7% chromium, 7.3% nickel, 1.0% aluminum, 0.6% silicon, 0.4%manganese, about 0.015% sulphur and phosphorus together, and remainderiron, was found by treatment in accordance with my invention to have theapproximate physical property values Ipresented below in the table. Allvalues tabulated for the annealed condition resulted from one-half hourheating of the steel at 1850 F. followed by quenching in water, thosefor the preliminarily hardened condition from heating the metal at 1400F. for three hours and Waterquenching, and values for the additionallyhardened condition resulted from heating the metal this being by heattreatment at temperatures o5 one hour at 900 F. and cooling in air.`

Table Ult. 0.29:7 P Tens. Yield Swen Femm Rockwell Condition Str" r.,Elozngfa. Hardness p.s.i. p. s. i.

annealed 143,500 43,100 23.5 58.5 B454 Preliminarnynardenemm. 143,100103,450 10.0 53.0 C-a sdditionauynardened 214,400 202,500 0.0 21.0 c-m'I'he close correlation of aluminum, a comparatively cheap material,with other elements in the steel accordingly enables a commerciallyvaluable precipitation hardening effect which becomes active with'proper treatment of the metal from a soft workable, or workedcondition. It will further be appreciated that the aluminum contributesto heat resistance and to the prevention of heat scale formation, asduring the respective hardening heats and in the resulting -hardenedcorrosion-resistant products.

Thus it will be seen that in this invention there is provided a methodand a chromium-nickel hardened stainless steel in which the variousobjects hereinbefore noted together with many other thoroughly practicaladvantages are successfully achieved. It will be seen that the methodmakes possible the provision of wrought or cast chromium-nickel-aluminumstainless steelrsubjected to any of a number of forming, machining orfabricating operations and eiectively and reliably hardened from a soft,ductile condition by heat treatment. Likewise, it will be seen that mymethod is readily practiced, and enables the production ofchromium-nickel grade stainless steel of hardened quality with a minimumof such treatments as pickling, and otherwise quite suitable forcommercial use. It will further be noted that the hardened productshavehigh yield and ultimate strengths, good directional properties, anda reasonable amount of ductility.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth, it is tobe understood that all matter described herein is to be interpreted asillustrative and not as a limitation.

I claim:

1. A chromium-nickel stainless steel susceptible to precipitationhardening by double heat treatment from a soft workable substantiallyfully austenitic condition, said steel containing chromium' and nickelin amounts substantially in accordance with area ABCD in theaccompanying` diagram, carbon between about 0.02% and 0.12%, about 0.50%to 2.50% aluminum, from incidental amounts up to about 8.0% manganese,from incidental amounts up to about 2.0% silicon, and the remaindersubstantially all iron.

2. A chromium-nickel stainless steel susceptible to precipitationhardening by'double heat treatment from a soft workable substantiallyfully austenitic condition, said steel containing chromium and nickel inamounts substantially in'accordance with area ABCD in the accompanyingdiagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50%aluminum, from incidental amounts up to about 8.0% manganese, fromincidental amounts up to about 2.0% silicon, at least one of the groupconsisting' 16.65% to 17.5% chromium, about 6.75% to 7.5% nickel,aluminum between about 0.60% to 1.0%, about 0.06% to 0.08% carbon, andthe remainder substantially all iron, the steel being precipitationhardenable by double heat treatment after the annealing.

4. In a method of hardening chromium-nickel stainless steel, providing asteel containing chromium and nickel in amounts substantially inaccordance with area ABCD in the accompanying diagram, aluminum inamounts between about 0.50% to 2.50%, about 0.02% to 0.12% carbon, fromincidental amounts up to approximately 8.0% manganese, from incidentalamounts up to about 2.0% silicon, and the remainder substantially alliron; then heating the steel at such temperature as to provide anaustenitic aluminum soluble condition stable down to at least about roomtemperature; transforming the alloy by heat treating and quenching; andheating the same at a temperature suiciently high and for such Iperiodof time to precipitate an aluminum compound and obtain a substantialincrease in the metal hardness.

5. In a method of hardening chromium-nickel stainless steel, correlatingabout 16.65% to 17.5% chromium, with about 6.75% to 7.5% nickel,aluminum in amounts between about 0.60% and 1.0%, about 0.06% to 0.08%carbon, and the remainder substantially all iron; treating said steelwithin a temperature range of about 1800 F. to 2000 F. to provide analuminum soluble austenitic condition stable down to at least about roomtemperature; transforming the alloy by heat treatment; and heating thetransformed alloy within a temperature range of approximately 750 F. to1000 F. for such period of time to precipitate an aluminum compound andobtain a substantial increase in the metal hardness.

6. In the production of hardened chromiumnickel stainless steel articlesand products. providing stainless steel containing about 16.65% to 17.5%chromium, with about 6.75% to 7.5% nickel, aluminum in amounts betweenabout 0.60% to 1.0%, about 0.06% to 0.08% carbon, and the remaindersubstantially all iron; annealing said steel within a temperature rangesufllciently high to provide an aluminum soluble austenitic conditionstable down to at least about room temperature; fabricating the articlesand products of said steel; reheating the fabricated metal within theapproximate temperature range of 1200 F, to 1600 F. to eiiect carbideprecipitation and quenching the same to achieve transformation; andheating the transformed metal within a temperature range ofapproximately 750 F. to 1000 F. for such period of time as toprecipitate an aluminum compound and obtain a substantial increase inhardness of the products and articles.

7. In the production of hardened chromiumnickel stainless steel articlesand products, providing stainless steel containing about 16.65% to 17.5%chromium, with about 6.75% to 7.5% nickel, aluminum in amounts betweenabout 0.60% and 1.0%, about 0.06% to 0.08% carbon, and the remaindersubstantially all iron; annealing said steel within a temperature rangesutilciently high to provide an aluminum soluble austenitic conditionstable down to at least about room temperature; reheating the steelwithin the approximate temperature range of 1200 F. to 1600 F. to effectcarbide precipitation, and quenching the same to achieve transformation;fabricating the articles and products of said steel, and heating saidfabricated metal within a temperature range of approximately 750 F. to1000" F. for such period of time as to precipitate an aluminum compoundand obtain a substantial in- ,crease in the metal hardness.

8. A chromium-nickel stainless steel precipita- 0.06% to 0.08% carbon,and the remainder sub- 5 stantally all iron.

9. A chromium-nickel stainless steel aluminumprecipitation hardened bydouble heat treatment from the annealed condition containing chromiumand nickel in amounts substantially in acl cordance with area ABCD inthe accompanying diagram, carbon between about 0.02% and 0.12%, about0.50% to 2.50% aluminum, from incidental amounts up to approximately8.0% manganese,

from incidental amounts up to about 2.0% silicon, l

and the remainder substantially all iron.

10. A chromium-nickel stainless steel aluminum-precipitation hardened bydouble heat treatment from the annealed condition, containing thechromium-like components aluminum in the amounts of about 0.50% to2.50%, silicon from incidental amounts up toY about 2.0% based on totalcontent of the steel, and the remainder substantially all chromium inamounts sufficient with chromium substitute for meeting'the terms for 2.

chromium of area ABCD in the accompanying diagram, said aluminum andsilicon serving as a substitute for chromium on about a 1 to 1 ratio;the nickel-like components carbon in the amount of about 0.02% to 0.12%,manganese from incidental amounts up to about 8.0%, and the remaindersubstantially all nickel in actual amount not less than about 3.5% andsufficient with nickel substitute for meeting the terms for nickel ofarea ABCD of the accompanying diagram, said carbon and manganese servingas a substitute for nickel on the ratios of about 1/20 to l/an partcarbon to 1, and approximately 2 parts manganese to 1; and the remaindersubstantially all iron, said aluminum being precipitated as aluminumcompound within the matrix of the steel.

11. A chromium-nickel stainless steel aluminumprecipitation hardened bydouble heat treatment from the annealed condition containing: thechromium-like components aluminum in the amount of about 0.50% to 2.50%,molybdenum from fractional percentages up to about 3.0%, silicon fromincidental amounts up to approximately 2.0% all based on total contentof the steel, and the remainder substantially all chromium in amountsuflicient with chromium substitute for meeting the terms for chromiumof area ABCD in the accompanying diagram, said aluminum, molybdenum andsilicon serving as a substitute for chromium on about a 1 to 1 ratio;the nickel-like components carbon in the amount of about 0.02% to 0.12%,manganese from incidental amounts up to about 8.0%, and the remaindersubstantially al1 nickel in actual amount not less than about 3.5% andsuiiicient with nickel substitute for substantially meeting the termsfor nickel of area ABCD in the accompanying diagram, said carbon andmanganese serving as a substitute for nickel on the ratios of about 1/zoto 1/:0 part carbon to 1, and approximately 2 parts manganese to 1; andthe remainder substantially all iron,` said aluminum being precipitatedto give substantial hardness.

GEORGE N. GOLLER.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date 1,943,595 Foley Jan. 16, 19342,048,163 Pilling July 21, 1938 2,384,567 Schaufus Sept 4, 19452,390,023 Wyche Nov. 27, 1945

10. A CHROMIUM-NICKEL STAINLESS STEEL ALUMINUM-PRECIPITATION HARDENED BYDOUBLE HEAT TREATMENT FROM THE ANNEALED CONDITION, CONTAINING THECHROMIUM-LIKE COMPONENTS ALUMINUM IN THE AMOUNTS OF ABOUT 0.50% TO2.50%, SILICON FROM INCIDENTAL AMOUNTS UP TO ABOUT 2.0% BASED ON TOTALCONTENT OF THE STEEL, AND THE REMAINDER SUBSTANTIALLY ALL CHROMIUM INAMOUNT SUFFICIENT WITH CHROMIUM SUBSTITUTE FOR MEETING THE TERMS FORCHROMIUM AREA ABCD IN THE ACCOMPANYING DIAGRAM, SAID ALUMINUM ANDSILICON SERVING AS A SUBSTITUTE FOR CHROMIUM ON ABOUT A 1 TO 1 RATIO;THE NICKEL-LIKE COMPONENTS CARBON IN THE AMOUNT OF ABOUT 0.2% TO 0.12,MANGANESE FROM INCIDENTAL AMOUNTS UP TO ABOUT 8.0%, AND THE REMAINDERSUBSTANTIALLY ALL NICKEL IN ACTUAL AMOUNT NOT LESS THAN ABOUT 3.5% ANDSUFFICIENT WITH NICKEL SUBSTITUTE FOR MEETING THE TERMS FOR NICKEL OFAREA ABCD OF THE ACCOMPANYING DIAGRAM SAID CARBON AND MANGANESE SERVINGAS A SUBSTITUTE FOR NICKEL ON THE RATIOS OF ABOUT 1/20 TO 1/30 PARTCARBON TO 1, AND APPROXIMATELY 2 PARTS MANGANESE TO 1; AND THE REMAINDERSUBSTANTIALLY ALL IRON, SAID ALUMINUM BEING PRECIPITATED AS ALUMINUMCOMPOUND WITHIN THE MATRIX OF THE STEEL.